sched/uclamp: Fix locking around cpu_util_update_eff()

cpu_cgroup_css_online() calls cpu_util_update_eff() without holding the
uclamp_mutex or rcu_read_lock() like other call sites, which is
a mistake.

The uclamp_mutex is required to protect against concurrent reads and
writes that could update the cgroup hierarchy.

The rcu_read_lock() is required to traverse the cgroup data structures
in cpu_util_update_eff().

Surround the caller with the required locks and add some asserts to
better document the dependency in cpu_util_update_eff().

Fixes: 7226017ad37a ("sched/uclamp: Fix a bug in propagating uclamp value in new cgroups")
Reported-by: Quentin Perret <qperret@google.com>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210510145032.1934078-3-qais.yousef@arm.com

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f97eb73..3ec420c 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9507,7 +9507,11 @@ static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
 
 #ifdef CONFIG_UCLAMP_TASK_GROUP
        /* Propagate the effective uclamp value for the new group */
+       mutex_lock(&uclamp_mutex);
+       rcu_read_lock();
        cpu_util_update_eff(css);
+       rcu_read_unlock();
+       mutex_unlock(&uclamp_mutex);
 #endif
 
        return 0;
@@ -9597,6 +9601,9 @@ static void cpu_util_update_eff(struct cgroup_subsys_state *css)
        enum uclamp_id clamp_id;
        unsigned int clamps;
 
+       lockdep_assert_held(&uclamp_mutex);
+       SCHED_WARN_ON(!rcu_read_lock_held());
+
        css_for_each_descendant_pre(css, top_css) {
                uc_parent = css_tg(css)->parent
                        ? css_tg(css)->parent->uclamp : NULL;